Method of vapor generation and vapor superheating, and apparatus therefor



Sept. 9, 1958 P. H. KOCH 2,851,015

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

' APPARATUS THEREFOR Filed Dec. s, 1955- 5 Sheets-Sheet 1 INVENTOR PAULh. K00.

Sept. 9, 1958 P. H. KOCH 2,851,015

METHOD OF VAPOR GENERATION AND.VAPOR SUPERHEATING, AND APPARATUSTHEREFOR Filed Dec. 3, 1953 5 Sheets-Sheet 2 INVENTOR PA M H Kocw i BYATTORNEY Sept. 9, 1958 P. H. KOCH v2,851,015

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

APPARATUS THEREFOR Filed Dec. :5, 1953 s Sheets-Sheet 3 & III N:

. I :3 1 I (Q'- I vC) I o O I O O O 6 1 I\\0 /9 OOOOOOKP/ Sept. 9, 1958P. H. KOCH ,8

METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND- APPARATUSTHEREFOR INVENTOR P414 kocy ATTORNEY P. H. KOCH Sept. 9, 1958 2,851,015METHOD OF VAPOR GENERATION AND VAPOR SUPERHEATING, AND

APPARATUS THEREFOR Filed Dec. 3, 1953 5 Sheets-Sheet 5 g i.m'

INVENTOR P404 H A ocH ATTORNEY United States Patent 0 METHOD OF VAPORGENERATION AND VAPOR SUPEATING, AND APPARATUS THEREFOR Paul H. Koch,Bernardsville, N. L, assignor to The Babcock & Wilcox Company, New York,N. Y., a-corporation of New Jersey Application December 3, 1953, SerialNo. 395,964

14 Claims. (Cl; 122-478) This invention relates to the art of vaporgeneration and vapor superheating, and the invention involves a par.-

ticular type of vapor generating and superheating unit, and a method ofoperation of such unit.

The invention is characterized by a vapor generating and superheatingunit and a method of operation of that unit, both including thesimultaneous effects of selective burner operation and correlated fiowof recirculated gas to modify furnace heat absorption and maintain apredetermined vapor superheat temperature over a Wide load range.

More specifically, as to the vapor generating and superheating unit ofthe invention, the pertinent unit involves a furnace fired by aplurality of burners (preferably, oil or gas burners) distributed over asubstantial proportion of the area of a boundary surface of the furnace.The furnace has vapor generating Wall tubes which normally absorb heatradiantly transmitted from the burner invention involves a convectionsection in whichzthe gases flow horizontally from the furnace andthrough successive gas passes. A convection superheater is disposed inone of the gas passes and it is connected to the vapor generating walltubes to receive generated vapor therefrom. The pertinent unit furtherinvolves agas recirculation system including a fan and duetworlcarranged to receive gases from a position beyond the super-heater and todischarge those gases in regulated quantities and at one or moreselected positions in the furnace, the openings through which therecirculated gases are discharged into the furnace being disposed in afurnace wall other than the furnace wall having the burners therein.Preferably, these openings are arranged in a furnaee wall opposite thewall of the burners.

The pertinent vapor generating and superheating unit involves a furnacegas exit opposite the burner wall and at a position successively moreremote from ditferent 2,851,015 1C6 Patented Sept. 9, 1958 When thepertinent vapor generating and superheating u is a e at no l fu ad a othe u bu ners a e i tende s be n QPQ Ta'F I h e rec tes- .lated gas intothe furnace isv at a minimum. As the dean f generated va er ecr a e nmet d f s eration of the pertinent unit involves the uniform decrease inthe rates of firing of all of the burners down toa predeterminedcapacity or rate of vapor generation. Durit s i ar at he lo n r an o gapq ne i apa t h n mal ten en at such a un t e deficient in superheatedvapor temperature is compensated or overcome by a regulated increase inthe how of recirculated gases into the furnace. This increase inrecirculated gas flow increasesthe heat absorbed by the superheater byreason of the increased mass flow of gases over the superheater, anddeereases the heat absorption bythe vapor generating wall tubes byreason of the pertinent manner of introduction into the furnace of thelower temperature recirculated gases. Both effects so change the ratioof vapor generating absorbedheat to superheater absorbed heat that thesuperheat temperature is 'ntained at a predetermined valueover thepertinent part of the load range. I

As the steam or vapor demand decreases through a part of the load rangenext lower than the load range mentioned immediately above, a pluralityof the burners are shutoff, these burners being disposed at positions moremote o he a exi of the fu es? than he remaining burners, and theremaining burners, continuing in opera n bei d pssed in a o ce t PQ tionwith respect to the furnace and, more particularly, with respect to thefurnace boundary in which all of the humus a di ed im a e us ith hi shutn fi 9 me of the r e s, h ir late as flow is' so directed into thefurnace and supplied thereto that there is an increase of the envelopingof the furnace heat emitting source by the lower temperaturerecirculated gases. Simultane ll l the distance from this heat emitn sur e t e of t e at a sor n l ube nc eased, an he hea e itt n sur a o t eh t emitting source is decreased. Allof these effects deerease the heatabsorbed in vapor generation. At the same time, the heat content in thegases leaving the furnace is of such increased value that the heatabsorbed by the superheater is increased. These effects are so regulated,as' to maintain a predetermined value of superheat temperature over thepertinent part of the load range. During successively lower parts of theload range all of the effects mentioned immediatelyabove are increasedby successively decreasing the number of operative burners andcontinuing the introduction of redreulated gases, until only one burnerremains in operation, with a maximum'of recirculated gas flow, a maximumdecrease'in the path of travel of combustion elements from the fuelburning means to the furnace exit, a maximum centralization of burningfuel projection into the furnace, a maximum envelopment of the projectedflame by the recirculated gases, and a minimum of surface of heatemitting source operative to provide heat for absorption by the vaporgenerating wall tubes of the furnace.

During the operative steps described above, the flow of recirculatedgases may not only be controlled as to quantity and velocity, but theirposition or positions of an optimum effect of all ofv the combined acts.For

example, when a plurality of the burners, more remote from the furnacegas exit are shut off, the furnace gas entry may be so controlled thatit is limited to a furnace position adjacent the location of the shutoff burners. In other words, when the recirculated gas flow into thefurnace takes place over any one of a plurality of positionssuccessively more remote from the furnace gas outlet, the furnace gasflow may be limited under the pertinent conditions to the position mostremote from the furnace gas outlet. Then, under the next successivesequence of acts, as the vapor generating load further decreases, thetype of conditions obtaining during the first load decrease may becontinued, and additional recirculated gases may be introduced at aposition more remote from the furnace gas exit at the same time thatadditional burners are shut off at positions more remote from thefurnace gas exit.

In conjunction with the above indicated subject matter, the inventionalso particularly contemplates an arrangement of oil or gas burners inan outside upright furnace wall, with the burners firing horizontally.With this arrangement, the recirculated gas system introduces partiallyheated gases through the rear wall of the furnace opposite the burnerwall and preferably slightly below the lower row of fuel burners. Therecirculated gases are directed horizontally forward through the furnacetoward the burner wall in proximity to the horizontally extendingfurnace floor. The floor preferably includes a skeleton of floor coolingsteam generating tubes connected into the circulation of the boiler andpreferably covered with ceramic refractory.

The utilization of natural gas as a fuel permits the furnace to be firedwith a sharp and substantially nonluminous flame. Luminosity will onlyoccur in case there is a cracking of some of the combustible elements todevelop carbon or some compound having radiant capabilities. In theburning of fuel oil the same high efliciency of combustion is notattainable in the same period of time, if at all, because the fuel oilmust not only be atomized but the small atomized particles of oil mustbe vaporized before they can combine with the oxygen of the combustionair. These actions increase the time involved in the combustion. Also,the combustion of fuel oil involves combustion products ofdifferent'characteristics. These products are luminous and have greaterradiating qualities than the products of natural gas combustion. Theirradiating qualities, however, are much less than the radiating qualitiesof the combustion products resulting from the burning of pulverizedcoal.

With oil or gas firing of the furnace, the relatively short flameburners may be located relatively close to the bottom of the burnerwall, inasmuch as there is no problem of cooling ash particles as wouldbe involved in the firing of the furnace by pulverized coal. With such ashort flame burner the furnace bottom disposed in proximity to theflames developed by the lowermost burners can be formed by ceramicrefractory material.

When such a furnace as that above described is operated in accordancewith prior art suggestions, and without recirculated gas, the intenseheat of the combustion zone developed by the lowermost burners resultsin radiant transfer of heat to the ceramic covering of the floor,bringing the face of the ceramic covering to a state of incandescence.In the present invention, providing for the innoduction of recirculatedgas horizontally across the furnace floor and toward the burner wall,the recirculated gases sweep the floor and absorb heat from the ceramicmaterial of the floor. This results in a decrease in temperature of theceramic floor covering so that the heat transferred from this coveringto the steam generating floor tubes is reduced, and the heat which wouldotherwise go into those tubes as a result of the incandescence of theceramic floor covering is,

lated gases and carried therewith to the furnace exit. The interposedstratum of recirculated gases absorbs radiated heat but it is still of atemperature lower than the furnace gases existing at that location whenthere is no recirculation. The radiant heat emission from the pertinentzone is reduced as is also the reradiation from the ceramic floorcovering. These are factors involved in obtaining a greater heat contentin the gases leaving the furnace. They also contribute to an increase inthe available convection heat in the gas pass beyond the furnace exit.

The introduction of the recirculated gases substantially horizontallyacross the furnace floor results in the continued flow of these gasestoward the burner wall, where they are deflected upwardly. This actiondeflects the burner flames and products of combustion of the short flameburners upwardly and, more specifically, diagonally across the furnacetoward the furnace exit. Also, the average velocity of the upwardlymoving gases is greater than would be the case if there were nointroduction of the recirculated gases. Thus, the residence time and theradiant transfer of heat from the gases to the furnace walls arereduced.

The invention will be specifically set forth in the claims appendedhereto, but for a more complete understanding of the invention and theadvantages and benefits attained by its use, reference should be had tothe accompanying description which refers to the pertinent drawingsdisclosing a preferred embodiment of the invention.

In the drawings:

Fig. 1 is a sectional elevation on the line 1--1 of Fig. 2, indicatingthe differential elevations of the rows of burners, and indicating thegas recirculation system;

Fig. 2 is a plan section taken on the line 22 of Fig. 1 showing therelation of the furnace to the serially connected gas flow passes of theconvection section;

Fig. 3 is a partial plan section taken on the line 3-3 of Fig. 1,showing the gas recirculation system and its gas outlets distributedover the length of the furnace;

Fig. 4 is a transverse sectional elevation taken on the line 4-4 of Fig.2, with the positions of the burners indicated in dash lines in aportion of the drawings in which the wall tubes are not shown;

Fig. 5 is a diagrammatic view similar to Fig. 4 but with the damperedgas outlets of the gas recirculation system shown and with the vaporgenerating tubes removed for the purpose of clarity; and

Fig. 6 is a chart or diagram intended to indicate the coordinated stepsof the illustrative method and the results produced thereby.

Figs. 1 and 2 of the drawings indicate the pertinent type of vaporgenerating and superheating unit. This unit is characterized by a boilersetting of rectangular outline including a front wall 10, a rear wall12, and

the connecting side walls 14 and 15. Parts of these walls, together withthe groups 17, 19 and 20 of upright vapor generating furnace wall tubes,and similar upright tubes 18 forming a vertical baffie extending fromthe wall 15 for a part of the width of the setting, constitute thefurnace or combustion chamber 22 fired by burners A-F inclusive,arranged in two substantially vertically spaced horizontal rows, asindicated in Fig. 4. Three of these burners are indicated in Fig. 2 atC, D and F.

From the furnace 22 the gases flow horizontally around the end of thebaflle 18 and through a plurality of gas passes 2426. In all of thesegas passes the gases flow horizontally over and between the spacedelements of banks of upright tubes 28-34, inclusive. The bank of tubes28 in the first pass 24 constitutes, with appropriate inlet and outletmanifolds, a convection superheater, and the banks of tubes 31-34, inthe gas passes 25 and 26, constitute vapor generating or fluid heatingtubes directly connecting the upper vapor and liquid drum 40 to a lowerdrum 42.

The first gas pass 24 is formed between the boiler setting wall 14. andabaflle 44, including appropriate heat resistant material between rows ofupright vapor generating, wall tubes 46 and 48. At the rear of the gaspass 24, the gases turn, as shown by the arrow 50, and proceed acrossthe banks of vapor generating tubes 31-32 to the gas turning space 52rearwardly of the baflle formed by the vapor generating tubes. 18. Fromthis gas turning space, the gases proceed rearwardly over the banks oftubes 33-34 to the space at the rear of the gas pass 26, from which, atleast a controlled proportion of the gases pass to a flue 53 or induceddraft fan (not shown). The second and third gas passes 25 and 26 areseparated by the baflle 54, including appropriate heat resistingmaterial between the rows of fluid heating tubes 56 and 58.

The furnace 22 extends. to an elevation below that of the drum 42, asindicated in Fig. 1,.the furnace; having a floor 60, including vaporgenerating floor tubes 62, connecting the front and rear floor headers64 and 66. The floor tubes are preferably covered with ceramicrefractory material 61 forming the upper surface of the furnace floor.Fig. 1 also clearly indicates the front wall vapor generating tubes,such as 20, as connecting to the header 64 and having upper partsconstituting roof tube sections 68 extending to connection with the drum40. The rear floor header 66 is similarly connected to the drum 40. bythe rear furnace wall tubes 70 some of which are included in the group18 of Wall tubes indicated in Fig. 2. The natural circulation circuitthrough the tubes 20. and 70 and the floor tubes 62 is completed bytubular connections 72 affording direct communication between the lowerdrum 42 and the header 66,.

The furnace side Wall vapor generating tubes 17 and 19 are connectedinto the natural circulation by' circulators such as 74. leading fromlower drum 42 to a side wall header 76 to which the tubes 17 aredirectly connected. The upper ends of these side wall tubes are directlyconnected to a header 78 which has direct connection with the drum 40.The vapor generating tubes 19 of the opposite furnace side wall aresimilarly connected into the natural circulation circuit through theheaders 80 and 82 and similar connections to the drums 40 and 42.

The superheater 28 is a convection superheater receiving vapor generatedin the unit and it is therefore subject to the inherent operativecharacteristic'that the vapor temperature at the outlet of thesuperheater decreases from an optimum value as the rate of vaporgeneration of the unit decreases, unless there is some compensatingoperative influence to prevent such drop in superheat (or superheatedvapor temperature at the outlet end of the superheater). In theillustrative unit such compensating influence is provided by a superheatcontrol which includes a plurality of coordinated factors. One of thesefactors is provided by the heating gas recirculation system including aninlet duct 90 particularly indicated in Fig. l as leading from an inlet92 at the bottom of the gas space disposed rearwardly of the last gaspass 26. This duct leads to the inlet of a gas recirculating fan 94, theoutlet of which is connected by duct 96 to a large manifold 98. Thismanifold has communicating therewith at distributed positions along itslength, the three lateral ducts 100402. These ducts communicate with thebottom of the furnace 22 through openings between the tubes 70, anddiflerential flow through the lateral ducts may be attained through theselective operation of louver type dampers 104106, a plurality of whichis disposed in each duct. The arrangement of the recirculated gas ducts100-102 and their outlets X, Y and Z with reference to the gas inlet tothe first gas pass 24 (I to IV inclusive) is particularly indicated inFig. 5, and it is to be noted that the gas recirculation outlets aredisposed at positions of successively different degrees of remoteness,relative to the inlet of the first gas pass 24 (or the furnace gasexit), I.IV inclusive. With this arrangement the heatabsorption byapproximately half load, as indicated by' LL.

the vapor generating tubes of the furnace may be, regulated bydifferential control of the recirculated gas flow through the outletsX",Y" and Z". If, for example, the heat absorption by the furnace wallvapor generating tubes is to be decreased to the maximum extent for flowthrough only one outlet, the flow of recirculated gas into thefurnace'may be limited to the outlet Z", with the dampers for theassociated outlets Y" and X" completely closed. Under such conditions,it is to be appreciated that the total flow of recirculated gas may notonly be regulated, but the time required by such gas flow to passthrough the furnace is increased by reason of the fact that the outlet Zis more remote to the furnace outlet, than either of the recirculatedgas outlets Y" or X". Under these conditions there would be a doubleeffect upon heat absorption by the furnace wall generating tubes, theone effect on heat absorption resulting from the closing of the,recirculated gas outlets Y" and X to reduce or limit thegtotal flow ofrecirculated gases; and the second effect being simultaneously operativeby reason of the increased time factor, relating to the period of timeduring which the furnace wall vapor generating tubes are subject to therecirculated gas flow.

The differential flow of recirculated gas through the outlets X", Y" andZ" may be also coordinated with the selective operation of the burnersA-F inclusive (Fig. 5). For instance (and particularly referring to Fig.6), the total flow through selected recirculated gas outlets may beincreased as the load decreases from a control point load (or full load)MM' to a value of This load decrease is indicated by the line ML' ofFig. 6. When the load value L-L is reached, the three burners, D, E andF, may be shut off. This action increases the centralization orconcentration of the ignition zone in a space immediately in front ofthe operating burners, relative to the surrounding heat absorbingsurfaces of the furnace, and thus decreases the heat emitting surface ofthe heat emitting source formed by the operating burners; and increasesthe heat radiation distance from the emitting source to at least some ofthe heat receptive furnace wall areas. Both of these effects tend toreduce the heat absorbed by the vapor generating tubes. These effects inreducing the furnace wall heat absorption may be also augmented by theconcentration or limitation of recirculated gas flow to the outlet Z ata position adjacent the furnace wall 15. Thus, a heat transfer retardingmedium of recirculated furnace gases would be interposed'between thewall 15 and the remaining operative burners A, B and C, furtherlimiting, for example, the radiant heat transmission from the morecentralized heat emitting source formed by the remaining operativeburners A, B and C.

The actions indicated immediately above might also, under some loadconditions, similarly result in a double or triple effect in reducingthe heat absorption by the vapor generating tubes of the floor of thefurnace. This might result from the introduction of lower temperature,and therefore, greater density recirculated gases in the lower part ofthe .furnace between the heat emitting source and the heat absorbingsurface formed by the floor tubes. This latter action might beparticularly increased when the recirculated gas outlet X" is closed andboth of the other outlets Y" and Z" are open.

When the three burners D, E and F are shut off as at point Y in Fig. 6,the recirculated gas fan capacity, or total flow of recirculated gasinto the furnace, is reduced as indicated by the line YX. Otherwise, thesuperheat might increase above the point L to an amount approximatelyindicated by the line LP, due to the tendency of the shortened distancefrom the heat emitting source to the furnace gas outlet to producehigher furnace gas temperatures at the outlet of the furnace, and, inpart, due to the tendency of the recirculated gas system to increase theflow of recirculated gases by reason of the reduced resistancein thatpart of the recirculated gas flow circuit leadingover' the convectionsurfaces.

7 It is contemplatedthat all six-burners will have their rateof firingreduced uniformly so that the total rate of heat input is reducedyasindicated by line MP as the load drops from M-M to LL, and as therecirculated g'as flow is increased, as indicated by the line ZY. As theload further drops from L-L' to KK, with only the three burners A, B andC in operation, the recirculated gas flow will be increased as indicatedby the line XW to increase superheat absorption by an increase of gasmass flow from the furnace and to effect an accompanying reduction infurnace wall absorbed heat in the manners above indicated.

When the load value KK is reached, the burner C is shut off, furthercentralizing the heat emitting source with reference to the roof, floorand side wall 15 of the furnace. In particular, the distance from heatemitting source to the floor of the furnace is increased to thusdecrease the furnace heat absorption for vapor generation. At the pointW the total flow of recirculated gas is decreased for the same reason asthe decrease in such gas flow at YX. Upon further decrease in loadtoward the value H-H, the superheat absorbed heat is increased by anincrease of gas mass flow caused by the increase of recirculated gasflow indicated by the line VU. This increase in recirculated gas flowsimultaneously decreases the furnace absorbed heat by aflfording agreater amount of thermal insulating medium between the remainingoperative burners A and B, and'the furnace heat absorbing surfaces,particularly the furnace floor.

When the low load value indicated by HE is reached the burner B is shutoff and only burner A remains in operation thus affording the maximum inthe reduction of a heat transmitting distance from a heat emittingsource to a heat absorbing wall surface or surfaces. At such load value,there is a maximum flow of recirculated gases into the furnace, whichmay be admitted through all of the recirculated gas outlets, or belimited to the outlets Y" and/ or Z only.

Certain features of my invention are disclosed in my prior copendingapplication, Serial No. 167,073, filed June 9, 1950, now U. S. Patent2,737,931.

Whereas the invention has been described with reference to the detailsof an illustrative embodiment, it is to be appreciated that theinvention is not limited to use in which all of those details areinvolved. The invention may rather involve the use of selected detailswith the omission of some of the remaining details. invention is to beconsidered as of a scope commensurate with the scope of the subjacentclaims.

What is claimed is:

1. In a natural circulation vapor generating and superheating unithaving a furnace including vapor generating tubes in its floor andwalls, a ceramic refractory covering for the floor tubes arranged toform a horizontally extending closed floor, short flame burners firingthe furnace from one wall at a position substantially above theelevation of the floor, a convection section including a convectionsuperheater heated by the gases from the furnace and disposed laterallyof the furnace for substantially horizontal flow of the gases over theelements of the convection section, and wide load range superheatcontrol means including a recirculated gas system including a fan andductwork leading from a gas flow position beyond the superheater torecirculated gas outlets disposed in a furnace wall opposite the burnerwall and directed toward the burner at a position close to and above thefurnace floor, said recirculated gas system directing recirculated gasesinto the furnace and across the floor in a direction opposite thedirection of fuel firing and at an elevation below thelowermostelevation 'offiring.' r t The 2. In a natural circulation vaporgenerating and superheating unit having a furnace with a substantiallyhorizontal closed floor including vapor generating tubes, means firingthe furnace from one wall at a position substantially above theelevation of the floor, a convection section including a convectionsuperheater heated by the gases from the furnace and disposed laterallyof the furnace for substantially horizontal flow of the gases over theelements of the convection section, and wide load range superheatcontrol means including a recirculated gas system including a fan andductwork leading from a gas flow position beyond the superheater andhaving recirculated gas outlets in a furnace wall opposite the wall ofthe firing means and just above the floor, said recirculated gas systemoutlets directing recirculated gases across the floor into the furnacein a direction opposite the direction of fuel firing and at an elevationbelow the lowermost elevation of firing.

3. In a steam generating and superheating unit, a steam and water drumat the upper part of the unit, a water drum at the lower part of theunit, groups of upright steam generating tubes connecting said drums, afurnace including some of said tubes as wall tubes, short flame fuelburning means including a plurality of fuel burners disposed atpositions at successively difierent degrees of remoteness from thefurnace gas outlet, means forming a convection gas flow path receivinggases from the furnace, a convection steam superheater in said path,means normally conducting steam from the steam and water drum to thesuperheater, furnace floor tubes arranged at a level below the level ofthe water drum, means connecting the floor tubes to said drums, aceramic refractory covering for the floor tubes, and wide load rangesuperheat control means including a recirculated gas system including afan and connected ductwork having an inlet communicating with the gasflow path beyond the superheater and having a plurality of outletsleading to the furnace at a position below the water drum, said outletsbeing disposed at positions of successively different degrees ofremoteness from the furnace gas outlet, the fuel burning means beingdisposed in a wall of the furnace opposite the water drum and at a levelabove the level of the outlets of the gas recirculation system.

4. In a steam generating and superheating unit, a steam and water drumat the upper part of the unit, a water drum at the lower part of theunit, groups of upright steam generating: tubes connecting said drums, afurnace disposed generally at one side of a plane including the drumcenterlines and including some of said tubes as wall tubes, fuel burningmeans including a plurality of fuel burners disposed at positions atsuccessively different degrees of remoteness from the furnace gasoutlet, means including a plurality of seriallyconnected horizontal gaspasses forming a convection gas flow path receiving gases laterally fromthe furnace, a convection steam superheater in the first of said gaspasses, means normally conducting steam from the steam and water drum tothe superheater, furnace floor tubes arranged at a level below the levelof the water drum, the floor tubes being connected to said drums, andwide load range superheat control means including a recirculated gassystem including a fan and connected ductwork having an inletcommunicating with the last of said gas passes beyond the superheaterand having a plurality of outlets leading to the furnace at a positionbelow the water drum, said outlets being disposed at positions ofsuccessively different degrees of remoteness from the furnace gasoutlet, the fuel burning means being disposed in a wall of the furnaceopposite the water drum and at a level above the level of the outlets ofthe gas recirculation system.

5. In a natural circulation steam generating and superheating unit, asteam and water drum at the upper part of the unit, a water drum at thelower part of the unit, upright steam generating tubes in communicationwith said drums,. a.furnace.including some of saidtubes as wall tubes,fuel burning means, means forming a convection gas flow path receivinggases from the furnace, a convection steam superheater in said path,means normally conducting steam from the steam and water drum to thesuperheater, furnace floor tubes arranged along a flat closed floor forthe furnace at a level below the level of the water drum, the floortubes being connected to said drums and to some of the furnace walltubes, and wide load range superheat control means including arecirculated gas system including a fan and connected ductwork having aninlet communicating with the gas flow path beyond the superheater andhaving an outlet leading to the furnace through a wall at a positionbelow the water drum, the fuel burning means being disposed in a wall ofthe furnace opposite the water drum and at a level above the level ofthe outlet of the gas recirculation system.

6. The combination of claim 5 further characterized by the dispositionof the recirculated gas system outlet leading through the furnace wallimmediately adjacent and parallel to the water drum.

7. The combination of claim 6 further characterized by a plurality ofseparately operable fuel burners arranged in succession from a wall ofthe furnace near one end of the water toward the opposite furnace wall,and a plurality of separately dampered recirculated gas flow openingsarranged in succession along the furnace wall immediately adjacent thewater drum and constituting the gas outlet of the gas recirculationsystem.

8. In a method of operating a natural circulation vapor generating andsuperheating unit having a plurality of fuel burners distributed over asubstantial part or area of a boundary surface of a furnace having vaporgenerating tubes along its walls, said burners being disposed atpositions successively more remote from the furnace gas exit, said unitalso having a convection vapor superheater receiving the vapor generatedin said tubes and heated by the gases from the furnace, the method ofcontrolling steam superheat temperatures over a relatively wide loadrange which comprises successively shutting off burners in sequence asthe demand for generated vapor decreases, simultaneously returning tothe furnace an increasing amount of gases which have been partiallycooled by passage over the superheater, said returning of partiallycooled gases also involving the entry of the partially cooled gases atdiffering degrees of remoteness from the furnace gas exit, saidsuccessive shutting off of burners also involving first shutting off aburner most remote from the furnace gas exit and then shutting off otherburners in sequence moving toward the burner nearest said exit, andreversing the direction or sequence of said acts as the demand forgenerated vapor increases toward a predetermined value.

9. In a method of operating a steam generating and superheating unitincluding a bent tube natural circulation steam generating systemincluding an upper steam and water drum and a lower water drum, saidunit including a plurality of serially connected horizontal gas flowpasses arranged in succession lengthwise of and between the levels ofthe drums, a furnace having steam generating tubes along its walls andhaving a furnace gas outlet leading to the first of said gas passes, aconvection superheater in one of said gas passes, fuel burning means atdistributed positions in a wall of the furnace opposite said furnace gasoutlet, said distributed positions having different degrees ofremoteness from said outlet, and wide load range superheat control meansincluding a gas recirculation system including a fan and ductwork havingsuccessive gas outlets leading into the furnace at a furnace wall otherthan the wall having the fuel burning means therein and having a gasinlet communicating with gas flow beyond the superheater, saidincreasing of the flow of recirculated gases as steam demand decreasesalso involving the variation of recirculated gas flow through saidsuccessive outlets at different degrees of remoteness from the furnacegas exit, said method comprising the steps su ccessively-shutting oflburners in sequence circumferentially of the area over which they aredistributed, as the demand for steam decreases, simultaneouslyincreasing the flow of recirculated gases, said successive shutting offof burners also involving first shutting off a burner most remote fromthe gas exit of the furnace and then shutting off other burners insequence moving toward the burner nearest said exit, and reversingsaidsequence or direction of said recirculated gas flow and fuel burningoperative acts as the steam demand increases.

10. In the natural circulation generation and superheating of ahigh-pressure elastic fluid; effecting the combustion of fuel to providehigh temperature gases; transmitting heat from the gases to confinedstreams of liquid to generate high-pressure elastic fluid; superheatingthe generated elastic fluid by the transmission of heat from the gasesafter loss of heat therefrom in the generation; and controlling thetemperature of the superheated elastic fluid over a wide range of therate of generation by withdrawing a percentage of the gases after lossof heat therefrom in superheating, introducing the withdrawn gases intothe combustion zone selectively at positions of different degrees ofremoteness relative to the gas exit of the combustion zone, andsimultaneously selectively varying the remoteness of the main combustionzone relative to the gas exit of the combustion zone.

11. In the natural circulation generation and superheating of ahigh-pressure elastic fluid; effecting the combustion of fuel by theprojection of fuel and air into a combustion zone to provide hightemperature gases; transmitting heat from the gases to confined streamsof liquid to generate high-pressure elastic fluid; superheating thegenerated elastic fluid by the transmission of heat from the gases afterloss of heat therefrom in the generation; and controlling thetemperature of the superheated elastic fluid over a wide range of therate of generation by withdrawing a percentage of the gases after lossof heat therefrom in superheating, introducing the withdrawn gases intothe combustion zone selectively at positions of different degrees ofremoteness relative to the gas exit of the combustion zone, andsimultaneously selectively varying the degree of remoteness of the maincombustion zone relative to the gas exit of the combustion zone, theintroduction of withdrawn gases being separate from the projection offuel and air into the combustion zone.

12. A steam generating and superheating unit comprising vertical walls,a horizontally arranged closed floor and a roof defining a setting ofrectangular horizontal crosssection, means forming a division wallarranged to divide said setting into a furnace chamber and a laterallyadjoining convection heating section communicating at one end thereof, anatural circulation steam generating system comprising an upperhorizontal steam and water drum and a lower horizontal water drumextending parallel thereto, a vertically arranged bank of steamgenerating tubes in said convection section connected to said upper andlower drums, and steam generating tubes arranged to cool vertical wallsand the floor of said furnace chamber, a convection heated steamsuperheater arranged in said convection section adjacent the gas inletend thereof, fuel burning means in one of said furnace chamber verticalwalls above the level of said floor, and means for controlling the finalsteam superheat temperature over a relatively wide load range includinga gas recirculating fan having its gas inlet connected to the gas flowpath in said convection section downstream of said superheater, andmeans for discharging recirculated gases substantially horizontallyacross said furnace chamber floor at a level below the lowermost fuelburning means.

13. The combination of claim 12 further characterized by the furnacefloor being disposed at a level substantially downwardly spaced from thelevel of the lower drum, the disposition of the recirculated gas systemas a plurality of separately dampered openings through the furnace wallReferences Cited in the file of this patent UNITED STATES PATENTS LuckeMay 31, Blizzard July 12, De Baufre Ian. 28, Kennedy July 29, Behr July24,

FOREIGN PATENTS Belgium June 30, Belgium Oct. 31,

